Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy, which typically uses a radiation treatment source (e.g., a linear accelerator (LINAC)) to generate radiation beams such as x-rays. In one type of external beam radiation therapy, a radiation treatment source (also referred to herein as a therapeutic radiation source) directs a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the therapeutic radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to and from the tumor. As a result, the cumulative radiation dose at the tumor is high and that to healthy tissue is relatively low.
The term “radiosurgery” refers to a procedure in which radiation is applied to a target region at doses sufficient to treat a pathology in fewer treatment stages (also known as treatment fractions or simply fractions) than with delivery of lower doses per fraction in a larger number of treatment stages. Radiosurgery is typically characterized, as distinguished from radiotherapy, by relatively high radiation doses per fraction or treatment stage (e.g., 500-2000 centiGray), extended treatment times per fraction (e.g., 30-60 minutes per treatment), and hypo-fractionation (e.g., one to five fractions). Radiotherapy is typically characterized by a low dose per fraction or treatment stage (e.g., 100-200 centiGray), shorter fraction times (e.g., 10 to 30 minutes per treatment) and hyper-fractionation (e.g., 30 to 45 fractions). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy (also referred to as x-ray therapy and radiation therapy) unless otherwise noted.
Image-guided radiation therapy (IGRT) systems include gantry-based systems and robotic arm-based systems. In gantry-based systems, a gantry rotates the therapeutic radiation source around an axis passing through the isocenter. Gantry-based systems include C-arm gantries, in which the therapeutic radiation source is mounted, in a cantilever-like manner, over and rotates about the axis passing through the isocenter. Gantry-based systems further include ring gantries having generally toroidal shapes in which the patient's body extends through a bore of the ring/toroid, and the therapeutic radiation source is mounted on the perimeter of the ring and rotates about the axis passing through the isocenter. Robotic arm-based systems include a robotic arm to which the therapeutic radiation source is mounted.
Associated with each radiation therapy system is an imaging system to provide in-treatment images (referred to herein as tracking images) that are used to set up and, in some examples, guide the radiation delivery procedure and track in-treatment target motion. Portal imaging systems place a detector opposite the therapeutic radiation source to image the patient for setup and in-treatment images, while other approaches utilize distinct, independent image radiation source(s) and detector(s) for the patient set-up and in-treatment images. Tracking images generated at some angles may be better suited to target tracking than tracking images generated at other angles. However, it can be difficult to determine which angles will produce the optimal target tracking performance.